Aqueous silicone dispersions cross linkable upon drying onto substrates into elastomeric films are known in the art. One type uses polydiorganosiloxanes as the primary ingredient, which is cross linkable through condensation and water removal. Such dispersions typically contain (i) an oil-in-water (O/W) emulsion of a hydroxy group containing polydiorganosiloxane such as an α,ω-dihydroxypolydimethylsiloxane, (ii) a condensation catalyst, (iii) a crosslinking agent, and (iv) an optional non-siliceous inorganic filler. Condensation catalysts most preferred are organo-metallic compounds such as dialkyltindicarboxylates. These hydroxy endblocked polydiorganosiloxane emulsions are generally produced by mechanically emulsifying pre-polymerized linear silicones with anionic and/or nonionic surfactants, or by means of emulsion polymerization of linear or cyclic oligomer siloxanes in anionic surfactant containing acidic systems.
The mechanism used for crosslinking generally falls into three main categories. In the first category, colloidal silica or an alkaline metal silicate is used as the crosslinking agent, and it is combined with an organic metal catalyst, as well as a high pH in the aqueous dispersion, to crosslink the polymer into an elastomer. In the second category, an alkoxysilane is used as the crosslinking agent, and the emulsion is filled with silica or a non-siliceous filler. In the third category, reactive silicone resins or metal siliconates are used to reinforce and/or crosslink an hydroxy endblocked polydiorganosiloxane.
Category one dispersions based on colloidal silica or silicates as crosslinking agents have inherent problems of limited shelf life. Certain measures can be taken to partially overcome this problem such as by aging the emulsion or by using divalent tin catalysts. Nevertheless, the storage stability problem is difficult to solve since neutral emulsions are not stable on storage, and do not cure to an elastomer after storage. The best-known technical solution involves storing the dispersions at a high pH, but high pHs can often be incompatible in certain applications. In addition, organic metal catalysts such as organic tin salts are necessary in curing the elastomeric film upon drying, and the presence of a metal catalyst is not always desirable in many final applications. Another inherent problem associated with dispersions containing colloidal silica or alkaline metal silicates is that the silica may agglomerate upon drying of the emulsion, forming an inhomogeneous film with inferior mechanical properties. Furthermore, there is evidence showing that silica particles in such dispersions reside at the silicone-water interface, with the result that there is less efficient crosslinking and reinforcing of the silicone than otherwise would be the case if the silica were distributed within the oil phase.
Category two dispersions necessarily produce an alcohol as a by-product volatile component. As a result, if significant amounts of alkoxysilane are incorporated, applications of the dispersion can be very limited. In addition, the alkoxysilanes are typically added to the aqueous phase, and have been shown to induce gelation of the emulsion if not added carefully.
In category three, the silicone resins or siliconates in such dispersions typically possess D(R2SiO2/2) units and T(RSiO3/2) units, or M(R3SiO1/2) units, D units and T unit constructions. In rare instances when a silicone resin containing M and Q(SiO4/2) units is used, it is generally in the form of a solution in toluene, again limiting their use in the market place. Alternatively when a Q unit containing resin is used, it often is a low molecular weight oligomer, e.g., a tetra-functional silane hydrolyzate, such that the resin can be solubilized in water and thus be incorporated into the emulsion as an aqueous solution or added directly to the aqueous continuous phase of the emulsion. The latter is exemplified in U.S. Pat. No. 6,521,699.
Emulsions according to this invention differ from known compositions in that they are prepared with silicone MQ resins for the crosslinking and the reinforcing of the linear hydroxy terminated polydiorganosiloxane in the formation of silicone MQ resin reinforced elastomeric films upon evaporation of water from the emulsion. These emulsions also contain large portions of silicone MQ resins and are free of metal containing catalysts. The compositions according to the invention provide many advantages including (i) good polymer stability both in the emulsion droplets and in the drying film, (ii) the crosslinking occurs sufficiently quickly into an elastomeric film by water evaporation under ambient conditions, (iii) dried films are macroscopically and microscopically homogeneous since the compositions undergo crosslinking and reinforcement via silicone MQ resins instead of via discrete silica particles, (iv) crosslinking and reinforcement occurs throughout the silicone oil phase and there is no domain segregation yielding intrinsically superior mechanical properties, (v) the emulsion is free of volatile organic compounds, (vi) the use of metal containing catalysts is eliminated, and (vii) it possesses a wide range of resin-to-linear ratio yielding a wide range of mechanical properties in the final product.